1. Field of the Invention
The present invention relates to novel aqueous polymer emulsions which are preferably partially or fully hydrolyzed and are particularly suited as wallcovering prepaste adhesives. More specifically, the invention comprises forming an aqueous emulsion of a vinyl ester with an amount of one or more water soluble monomers to yield a product which has adhesive properties. Particularly preferred emulsions have a viscosity of less than 1000 centipoises.
2. Technology Description
A number of materials have been proposed for wallcovering paste materials, and particularly for wallpaper prepaste adhesives. A wallpaper prepaste adhesive is a material which is applied to one side of the wallpaper substrate and then is dried on the substrate. To adhere the paper to a wall, the substrate side with the prepaste adhesive, or more commonly, the entire wallpaper sheet is moistened, typically with water, to cause the adhesive material to swell. The adhesive side is applied to the surface to be covered. An alternative method of applying wallpaper to a surface is to use a "brush on" adhesive which is applied to an uncoated surface of the paper just prior to application of the paper to the wall. While both methods for applying wallpaper to a wall are used, the wallpaper prepaste method is far more common.
Known prepaste adhesives for wallcoverings include starch based materials, modified cellulose materials, alkaline-soluble oil-in-water (O/W) emulsion materials and crosslinked acrylamide-sodium acrylate materials in a water-in-oil (W/O) emulsion. In selecting a prepaste adhesives, the material selected should possess the following attributes: can be applied in one pass, is environmentally safe, is cost effective, is easy to handle, is non-toxic, demonstrates good paste properties and is compatible with commercially available coating machines. Despite the success of the above materials, none can satisfy all of the above criteria
A representative example of a starch based adhesives is described in U.S. Pat. No. 3,696,065. Starch adhesives are typically applied to wallcoverings in either powder or aqueous form. These materials can be environmentally deleterious so they can produce dust upon application to the substrate and when applying the covering to a surface. Further, starch based adhesives are not necessarily heat stable and as such are limited in their use. For example, they cannot be used in applications which involve hot embossing procedures. U.S. Pat. No. 3,696,065 specifically discloses the addition of a small amount of a vinylpyrrolidone/vinyl acetate solid copolymer to the starch to improve its adhesiveness.
A representative example of a modified cellulose based adhesive is described in U.K Published Application No. 2,054,628. Modified cellulose adhesives are typically applied to wallcoverings in either powder or aqueous form. These materials can be environmentally deleterious as they can produce dust upon application to the substrate and when applying the covering to a surface. Further, modified cellulose based adhesives are not necessarily heat stable and as such are limited in their use. For example, they cannot be used in applications which involve hot embossing procedures. These materials often do not provide as much adhesion as desired.
A representative example of an alkaline-soluble oil-in-water emulsion type adhesive is described in U.S. Pat. No. 4,741,790. To produce a wallcovering having one side coated with the prepaste adhesive, a two pass coating technique is used. In a first pass, an ionic material which is acidic in nature is coated onto a substrate. Thereafter, in a second pass a layer of base, typically Na.sub.2 CO.sub.3 is coated over the initial coating to neutralize the acid present in the ionic material. This method, while practiced commercially, suffers in that it requires two precise coating passes to yield a satisfactory final product. This can prove to be both cumbersome and costly.
A representative example of a cross-linked acrylamlde-sodium acrylate polymer material in a water in oil emulsion is described in Canadian Patent No. 1,304,185. The adhesive polymeric materials are coated onto a substrate in a mineral spirit solvent. While such materials have gained wide commercial acceptance, they contain an organic solvent, which causes the volatile organic content (VOC) of the resulting emulsion to be higher than ideally desired. As such, these organic solvent-based materials are subject to environmental scrutiny.
Hybrids of the above-described prepaste adhesives have been proposed. For example, U.S. Pat. No. 4,719,264 teaches an adhesive composition made from a blend of an aqueous emulsion of a polymer of vinyl acetate, an organic solvent solution of a copolymer of vinyl acetate and vinyl pyrrolidone having dextrin dispersed therein and an aqueous emulsion of an ester gum tackifier. Such materials still face because of the VOC issue. Further, the starch present in these materials can cause the above-described heat stability problem. Accordingly, these may not be ideal candidates for prepaste adhesives.
Copolymers and blends of homopolymers of vinyl acetate with one or more monomers are known for a variety of uses. For example, U.S. Pat. No. 3,345,320 discloses a solid vinyl pyrrolidone/vinyl acetate copolymer which, when combined with a plasticizer, is useful as a hot melt adhesive. U.S. Pat. No. 5,122,582 discloses the use of a clear aqueous solution containing a copolymer of N-vinyl pyrrolidone and vinyl acetate which is useful as a hair setting lotion or gel. The article "Polymers of N-Vinylpyrrolidone: Synthesis, Characterization and Uses", "Polymer Journal, Vol. 17, No. 1, pp. 143-152 (1985), suggests that copolymers of N-vinylpyrrolidone can be used as adhesive sticks and as re-moistenable adhesives. The article fails to disclose the use of such copolymers in aqueous emulsion form.
U.S. Pat. Nos. 3,166,525 and 3,691,125 disclose aqueous emulsions of copolymers of vinyl acetate with N-vinyl pyrrolidone prepared in the absence of an emulsifier or protective colloid. U.S. Pat. No. 3,166,525 suggests a number of uses for the emulsions, or the copolymers recovered after removal of water including textile and other finishes, anti-snag finishes for hosiery, sizings for cloths and fibers and as adhesive base materials for paper, plastics, metals, ceramics and the like. According to the examples, the viscosity of the emulsions produced are extremely high (80,000 cps). As a result, the emulsions could not be used as wallpaper prepaste adhesives as it would be impossible to use them with commercial coaters.
U.S. Pat. No. 3,691,125 produces the above copolymers in emulsion form and uses an amount of a poly-N-vinyl lactam, preferably poly vinylpyrrolidone as a seed material. According to the examples, the viscosity obtained when producing a copolymer of vinyl acetate and N-vinyl pyrrolidone ranges from about 13,200 to about 14,700 cps (Example 1) and about 4060 cps (Example 2). These emulsions could not be used as a wallpaper prepaste adhesive as it would be impossible to use them with commercial coaters.
U.S. Pat. No. 4,251,400 discloses adhesives based upon polyvinyl acetate. More specifically, the adhesive comprises an emulsion blend of polyvinyl acetate, polyvinylpyrrolidone and a plasticizer. The adhesive is suggested for use by children as it can easily be removed from clothing. This product cannot swell in water, and as such, is not useful as a prepaste.
Despite the above, there still exists a need in the art for an environmentally safe aqueous emulsion which can be used as a wallcovering prepaste adhesive.